1. Field of the Invention
The present invention relates to a thick film resistor composition used for electronic parts such as a highly integrated circuit, a chip, a volume, or a high voltage resistor, more specifically to the thick film resistor composition having improved temperature dependency of resistance value and reduced current-noise.
2. Description of the Related Art
A thick film resistor composition used for electronic parts is made in the form of paste, which is made by mixing fine conductive powder such as RuO.sub.2 and non-conductive glass powder with organic vehicle. The paste is applied to a ceramic substrate by using, for example, screen printing, and is fired at a predetermined temperature to form a resistor coating on the ceramic substrate.
The temperature coefficient of resistance (which is referred to as TCR hereinafter) and current-noise (which is referred to as noise hereinafter) are important characteristics of the thick film resistor composition produced as set forth above.
Practically, the TCR is represented by the rates of change which are referred to as "cold or low temperature coefficient (CTCR)" and as "hot or high temperature coefficient (HTCR)." The CTCR is the rate of change of resistance values at low temperature (at -55.degree. C.) generally expressed using values per 1.degree. C. (ppm/.degree.C.) on the basis of resistance value at 25.degree. C. as shown in the following expression Eq.1, and the HTCR is the rate of change of resistance value at high temperature (at 125.degree. C.) generally expressed using values per 1.degree. C. (ppm/.degree.C.) on the basis of resistance value at 25.degree. C. as shown in the following expression Eq.2.
Preferably, the TCR value becomes 0 ppm/.degree.C. ##EQU1## where R.sub.-55 is a resistance value (.OMEGA./.quadrature.) at -55.degree. C., R.sub.25 is a resistance value (.OMEGA./.quadrature.) at 25.degree. C. and R.sub.125 is a resistance value (.OMEGA./.quadrature.) at 125.degree. C.
On the other hand, the noise is the current-noise occurring in the thick film resistor, and is measured by a Quan Tech noise meter. Preferably, the noise becomes as small a value as possible.
In order to obtain the TCR value as close as possible to zero, the thick film resistor has been improved by adding various types of inorganic compounds thereto. Various inorganic compounds are disclosed in, for example, Japanese Patent Application Laid-Open No. 48-82391, Japanese Patent Publication No. 55-39883 and Japanese Patent Publication No. 54-1917, Japanese Patent Application Laid-Open No. 47-8579, and Japanese Patent Publication No. 57-26401. In these publications, negative TCR adjustors such as Nb.sub.2 O.sub.5, TiO.sub.2, MnO.sub.2 or Sb.sub.2 O.sub.3, and positive TCR adjustors such as CuO are employed as additives.
Further, it is necessary to reduce an addition rate of the fine conductive powder in order to obtain a thick film resistor composition having higher sheet resistivity (.OMEGA./.quadrature.).
However, this increases the noise. Therefore, as disclosed in Japanese Patent Application Laid-Open No. 48-82391 and Japanese Patent Application Laid-Open No. 47-8579, the sheet resistivity has been increased while maintaining a higher addition rate of the fine conductive powder in the mixture by adding Nb.sub.2 O.sub.5, Sb.sub.2 O.sub.3 or the like.
However, there has been a problem in that the above conventional method tends to provide negative TCR.